1. Field of the Invention
The present invention relates generally to navigation receivers, and more specifically to a method and apparatus for performing Doppler and code phase searches in a Global Navigation Satellite System (GNSS) receiver.
2. Related Art
Global Navigation Satellite System (GNSS) generally refers to satellite-based navigation systems, which provide geo-spatial positioning capabilities (i.e., determination of position). As is well known in the relevant arts, in a GNSS system, satellites (GNSS transmitters) transmit GNSS signals carrying ranging information, correction parameters, etc., that enable a receiver capable of receiving GNSS signals (GNSS receiver) to compute its position. Global Positioning System (GPS), WAAS (Wide Area Augmentation System), Galileo System are some examples of GNSS.
Typically, a GNSS signal contains a data combined with a code, with the combination modulating a carrier. In a GNSS system, multiple satellites may be present, with each transmitting a GNSS signal having a same carrier frequency, but different codes and data. In general, each transmitter is assigned a corresponding different code. Thus, a received GNSS signal (at a GNSS receiver's antenna, for example) may contain one or more of the transmitted GNSS signals (transmitted by respective transmitters).
In order to recover data from the respective transmitted signals contained in the received GNSS signal, a GNSS receiver generally needs to search the corresponding transmitted signals, a procedure often termed signal acquisition and then lock onto them, and to track the further changes, referred to as tracking. A procedure often implemented in a GNSS receiver to acquire/track a transmitted signal entails correlating a down-converted (often at baseband) received GNSS signal with a corresponding local signal (generated within the receiver).
Correlations are often performed for code phase searches and carrier Doppler searches. As is well known, a code phase search implies determining the specific time instance (or point on the received signal) at which each code commences.
On the other hand, a Doppler search is directed to determination of the carrier frequency. While the applicable standards specify an ideal carrier frequency, the actual frequency with which the signal is received can change due to Doppler effect. As is well known in the relevant arts, due to relative motion between a GNSS receiver and a satellite, the carrier frequency of the transmitted signal may deviate from a nominal value (ideal carrier frequency) due to the Doppler effect.
It is generally desirable that the code phase and Doppler searches be performed while meeting the various requirements as suited to the specific environments of deployment.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.